This invention relates to a process for the production of adhesives and more particularly, it relates to a multi-stage irradiation process for the production of acrylic-based adhesives, especially pressure-sensitive adhesive tapes. It also relates to acrylic-based adhesives and tapes made by the novel processes of this invention.
It is known in the literature and in the industry that there are at least four different feasible methods for the production of acrylic-based pressure-sensitive adhesive (hereinafter xe2x80x9cpsaxe2x80x9d) tapes. These known methods include solution polymerization, emulsion polymerization, irradiation by high energy particulate matter (e.g., electron beams or gamma rays), and ultraviolet light (hereinafter xe2x80x9cUVxe2x80x9d) photopolymerization. As explained below, however, there are disadvantages and/or limitations incurred with the use of each known process.
When utilizing psa""s made by solution polymerization, elaborate drying ovens with massive exhaust ducts and high temperatures are required to carry away the volatile solvents after coating. Furthermore, to prevent the solvents from being vented to the atmosphere (with resulting pollution and solvent loss), expensive solvent recovery equipment has been necessary. Safety hazards in such operations are also severe, as the solvents are extremely flammable and precautions must be taken to avoid explosive mixtures in the oven and exhaust systems. A further limitation of the solvent based systems is the limit on the thickness of the coatings which can be deposited in one trip or pass through the coater. Thus, with coatings above about 5 mils, multiple coating layers must be deposited in successive trips through the coater to avoid blistering of the coating due to solvent evaporation.
While emulsion polymerization has eliminated the problems associated with the handling and evaporation of flammable solvents, the heat of vaporization must be supplied to remove the water from the coating and essentially the same equipment must be employed. Though higher solids coatings are possible, the higher heat of vaporization of water as compared to organic solvents offsets this benefit and about the same total energy for drying is required. Drying times are relatively long, thus limiting production. One of the most serious limitations of the emulsion polymerization process is the water sensitivity of the resulting polymers (caused by the emulsifying agent which is carried along in the process and becomes part of the final polymer). A further limitation of this process is that highly polar monomers, which are water miscible, are difficult to incorporate into the copolymer during polymerization and considerable homopolymerization of such monomers can occur in the aqueous phase.
Various attempts have been made to avoid the difficulties of the solution and emulsion polymerization processes. However, those have thus far resulted either in processing difficulties of their own or have produced polymers in which a proper balance between compliance and cohesive strength is very difficult to control.
More recently, development work has been done with polymerization processes which employ either ultraviolet light or electron beams. One patent which stresses electron beam curing is U.S. Pat. No. 3,897,295, in which the composition subject to the electron beam includes an acrylate monomer selected from a particular specified group, and a homopolymer or copolymer of a substance or substances selected from the same group. The polymer is dissolved in the monomer and the monomer is ultimately polymerized to bind the adhesive together.
The disadvantage of utilizing polymerization processes involving an electron beam, though, is that, generally, it is a rather indiscriminate polymerization process. In polymerization processes utilizing an electron beam, the particulate bombardment of the free-radically polymerizable monomers cannot be precisely controlled, with the result being chain scission of the developing polymer and an inability to control its molecular weight and crosslink density to the most desired range.
In order to avoid the above-discussed disadvantages incurred with the use of an electron beam, some have chosen to utilize a one step or stage low-intensity (e.g., 0.1 to 7 mW/cm2) UV photopolymerization process. See, for example, U.S. Pat. No. 4,181,752. Whereas the use of relatively low intensity UV light is very desirable for building higher molecular weight acrylic psa""s with good performance properties, an increase in the speed of the photopolymerization process would be desirable. However, if one attempts to increase the speed of the low intensity UV light-based process by increasing the amount of the photoinitiator employed (e.g., benzoin ethers, benzil ketals, etc.), then undesirable lower molecular weight polymers will be obtained. Furthermore, for thick adhesives an uneven polymerization from the front surface to the back surface of an irradiated adhesive composition occurs due to the uneven light absorption by the photopolymerization initiator resulting in a differential performance of the final psa product.
In view of the foregoing discussed disadvantages and limitations that exist with the use of conventional polymerization processes, improvements are continuously desired and sought by those within the industry. It was against this background that an improved polymerization or irradiation process for producing acrylic-based adhesives, and in particular acrylic-based psa tapes was sought.
By the present invention, we have now discovered that an increase in the speed of the photopolymerization process for the production of acrylic-based adhesives and acrylic-based psa tapes with acceptable product properties can be achieved by utilizing a multi-stage (as opposed to the conventional one-step) irradiation process, the first stage employing electromagnetic radiation at a relatively low intensity and the subsequent stage employing electromagnetic radiation at a relatively higher intensity.
Accordingly, the inventive multi-stage irradiation process for the production of an acrylic-based adhesive comprises the sequential steps of:
(a) forming a monomeric mixture or partially prepolymerized syrup comprising:
(i) 50-100 parts by weight of at least one acrylic acid ester of an alkyl alcohol, the alcohol containing from 1 to 14 carbon atoms;
(ii) 0-50 parts by weight of at least one copolymerizable monomer; and
(iii) a photoinitiator;
(b) irradiating the resulting monomeric mixture or syrup with electromagnetic radiation of from 280 to 500 nanometer wavelength and from 0.01 to 20 mW/cm2 average light intensity to effect conversion of from about 5-95 wt % of the monomeric mixture or partially prepolymerized syrup to an acrylic copolymer; and
(c) thereafter, irradiating the acrylic copolymer resulting from step (b) with electromagnetic radiation of from 280 to 500 nm wavelength and having an average light intensity of greater than 20 mW/cm2 to at least substantially complete the photopolymerization reaction of the acrylic copolymer.
In another embodiment of the present invention, pressure-sensitive adhesive tapes are produced by coating the monomeric mixture or prepolymerized syrup onto a substrate and thereafter, irradiating the coated substrate as per steps (b) and (c) of the foregoing disclosed process.
In still other embodiments of the present invention, acrylic-based adhesives and psa tapes are provided which are produced by the foregoing disclosed, inventive processes.
The inventive multi-stage irradiation process does not have the limitations and drawbacks discussed earlier herein associated with conventional polymerization processes. The fact that the inventive process employs sequential irradiation steps or stages, one involving exposure to electromagnetic radiation at low average intensity and a subsequent stage or step involving exposure to electromagnetic radiation at a higher average intensity, is significant because the process is not only highly efficient, but it also produces an acrylic-based psa product having excellent properties, e.g. high shear strengths and adhesion values, as well as adhesive products and other articles which are not pressure-sensitive at room temperature.
Other acts, advantages, and benefits of the present invention are apparent from the detailed disclosure, examples, and claims.
In the first step of the inventive process, a photopolymerizable monomeric mixture or partially prepolymerized syrup is made by mixing together the components described below.
The photopolymerizable monomeric mixture comprises 50-100 parts by weight of at least one acrylic acid ester of an alkyl alcohol (preferably a non-tertiary alcohol), the alcohol containing from 1 to 14 (preferably 4 to 14) carbon atoms. Included within this class of monomers are, for example, isooctyl acrylate, isononyl acrylate, 2-ethyl-hexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl acrylate, methyl acrylate, and hexyl acrylate. Preferred monomers include isooctyl acrylate, isononyl acrylate, and butyl acrylate, and 2ethyl-hexyl acrylate.
The acrylic acid ester (xe2x80x9cacrylatexe2x80x9d) is polymerized with 0-50 parts of at least one copolymerizable monomer which is typically an ethylenically unsaturated polar monomer such as, for example, acrylic acid, methacrylic acid, acrylamide, acrylonitrile, methacrylonitrile, N-substituted acrylamides, hydroxy acrylates, N-vinyl lactam, N-vinyl pyrrolidone, maleic anhydride, isobomyl acrylate, and itaconic acid.
In a preferred embodiment of the present invention, the viscosity of the monomer mixture is increased to a more desirable level so that it can be handled more conveniently during the coating process. In order to increase the viscosities to a more desirable level, the monomer mixtures can be partially prepolymerized. The prepolymerization can be accomplished by exposure to electromagnetic radiation (such as UV light) or by thermal polymerization. Other methods of increasing the viscosity of the monomer mixture are also available, however, such as the addition of viscosity modifying agents such as high molecular weight polymers or thixotropic agents such as colloidal silicas, etc. A syrup is a monomeric mixture thickened to a coatable viscosity.
If a psa tape is to be made, the monomeric mixture or prepolymerized syrup can be coated onto any suitable substrate (e.g., backing or liner) including, for example, films (transparent and non-transparent), cloths, papers, non-woven fibrous constructions, metal foils, and aligned filaments.
Afterwards, the monomeric mixture or partially prepolymerized syrup is photopolymerized by irradiating the same with electromagnetic radiation of 280 to 500 nanometer wavelength and 0.01 to 20 mW/cm2 average light intensity to effect about 5-95% conversion of the monomeric mixture or prepolymerized syrup to an acrylate copolymer.
The irradiation for both stages is preferably carried out in the absence of oxygen. Thus, it is normally carried out in an inert atmosphere such as nitrogen, carbon dioxide, helium, argon, and the like. Air can also be excluded by sandwiching the liquid polymerizable mixture between layers of solid sheet material and irradiating through the sheet material. As will be appreciated by those skilled in the art, such material can have low adhesion surfaces and can be removed after polymerization is complete or one such surface can be a tape backing material. Preferably, the stages of irradiation are conducted continuously, or in-line without interruption of the polymerization process, i.e., the coated mixture is exposed to the first stage irradiation and then immediately exposed to the second stage irradiation with no interruption of the inert atmosphere between the stages.
Photoinitiators useful in the present invention include, but are not limited to, the benzoin ethers, substituted benzoin ethers such as benzoin methyl ether or benzoin isopropyl ether, substituted acetophenones such as 2,2-iethoxy-acetophenone, and 2,2-dimethoxy-2-phenyl-acetophenone, substituted alpha-ketols such as 2-methyl-2-hydroxypropiophenone, aromatic sulphonyl chlorides such as 2-naphthalene sulphonyl chloride, and photoactive oximes such as 1-phenyl-1,2-propanedione-2-(O-ethoxycarbonyl) oxime, and the substituted triazines. Preferably, the photoinitiator is present in an amount of from about 0.01 part to about 5 parts, and most preferably, about 0.10 to 2 parts by weight, based upon 100 total parts by weight of monomer.
The photopolymerizable composition preferably contains a crosslinking agent to enhance the cohesive strength of the resulting adhesive or article. Useful crosslinking agents which also function as photoinitiators are the chromophore-substituted halomethyl-s-triazines disclosed in U.S. Pat. Nos. 4,330,590 and 4,329,384. Other suitable crosslinking agents include hydrogen abstracting carbonyls such as anthroquinone and benzophenone and their derivatives, as disclosed in U.S. Pat. No. 4,181,752, and polyfunctional acrylates such as 1,6-hexanedioldiacrylate, trimethylolpropane triacrylate and 1,2-ethylene glycol diacrylate, as well as those disclosed in U.S. Pat. No. 4,379,201.
Where a foam-like material or foam psa adhesive tape is desirable, a monomer blend comprising microbubbles may be used. The microbubbles may be glass or they may be polymeric. The microbubbles should have an average diameter of 10 to 200 micrometers, and comprise from about 5 to about 65 volume percent of the pressure-sensitive adhesive layer. Preferred glass microspheres have average diameters of about 50 micrometers. Especially preferred microspheres are polymeric microspheres, such as those disclosed in U.S. Pat. Nos. 3,615,972, 4,075,238, and 4,287,308.
The adhesives and psa tapes made by utilizing the processes of this invention may include cellular pressure-sensitive adhesive membranes as discussed in U.S. Pat. No. 4,415,615.
A foam-like or cellular psa membrane can be made by the process of the present invention. Typical psa thickness may range from 0.3 to 4 mm.
Other materials which can be blended with the polymerizable monomer mixture include fillers, tackifiers, foaming agents, antioxidants, plasticizers, reinforcing agents, dyes, pigments, fibers, fire retardants, and viscosity adjusting agents.
An especially useful filler material is hydrophobic silica as disclosed in U.S. Pat. Nos. 4,710,536 and 4,749,590. In one preferred embodiment of the present invention, the pressure-sensitive adhesive layer further comprises from about 2 to about 15 parts per hundred parts (phr) of a hydrophobic silica having a surface area of at least 10 m2/g.
The extent of polymerization can be monitored by measuring the refractive index of the adhesive layer. Refractive index is a sensitive measure of the extent of polymerization and for these systems changes from about 1.4300 at the syrup stage to about 1.4700 at 100% reaction. The change in refractive index occurs linearly with conversion of acrylate unsaturation. This method is commonly applied in polymerization kinetics work. See, for example, discussions about the method in Polymerization at Advanced Degrees of Conversion, G. P. Gladyshev and K. M. Gibov, Keter Press, Jerusalem, 1970.
The subsequent irradiation is carried out at 280 to 500 nm wavelength and at a higher average intensity (as compared to the first stage irradiation step) within the range of greater than 20 mW/cm2; preferably in the range of greater than 20 up to about 200 mW/cm2; and most preferably in the range of about 50 mW/cm2 to 150 mW/cm2. During the second stage irradiation, the copolymerization reaction is at least substantially completed (i.e. preferably at least 95 wt % monomer conversion has been achieved as well as any crosslinking) thereby forming the acrylic copolymer. In some instances it is desirable to cool the adhesive coated web during both stages of irradiation, and especially during the second stage irradiation. This can be done by pulling the adhesive coated film over a cooling plate, by blowing cooled nitrogen on the web, or by blowing cooled air onto the aforementioned sandwich constructions.
The first stage of the irradiation uses low average intensity light (0.1-20 mW/cm2) obtained from, but not limited to, fluorescent black lamps. The type of lamps that are useful in the practice of the invention depend upon the photoinitiator that is used. The lamps should emit radiation at wavelengths needed to activate the photoinitiator. For example, fluorescent black lamps having an output between 280-500 nm are useful. For convenience, fluorescent black lamps having an output between 300-400 nm are preferred. The subsequent polymerization step utilizes an average light intensity of greater than 20 mW/cm2 obtained from high pressure or medium pressure mercury, mercury xenon, or similar type doped lamps which operate at 100-600 watts/inch input power. Intensities of up to about 200 mW/cm2 are feasible with the state of the art lamps. Higher intensities may be possible as advancements in bulb technology are made.
Each stage of irradiation can further include zones of varying intensities within the ranges specified, i.e., the first stage irradiation can include zones having varying low average intensity light, and the second stage can include zones having average light intensities greater than 20 mW/cm2.
In addition to conducting the second stage polymerization with high intensity electromagnetic radiation, it is also feasible to utilize high energy particulate matter such as, for example, electron beams.
Utilizing the process of this invention substantial increases in processing speeds can be obtained while maintaining high performance psa properties.
The following non-limiting examples further illustrate the present invention. The test procedures used for evaluating the tapes of the Examples are given below.
180xc2x0 Peel
A sample is prepared by cutting an adhesive-coated polyethylene terephthalate film into a strip 2.54 cm by 15.2 cm. The strip is then laminated to a smooth stainless steel panel and rolled down with 2 passes of a 2.04 kg. hard rubber covered roller. The sample is left at room temperature for 15 minutes. The tape is then pulled away from the stainless steel panel at an angle of 180 degrees and a speed of 30.5 cm per minute on a tensile tester. Results are reported in Newtons/dm.
Static Shear
A sample is prepared by attaching a 2.54 cm wide by 15.2 cm long adhesive coated polyethylene terephthalate film strip to a smooth stainless steel panel that has been cleaned with heptane. The adhesive coated strip is overlapped onto the panel a distance of 2.54 cm so that the total contact area of the adhesive coated film on the stainless steel panel is 2.54 cm by 2.54 cm. The overlapped area is then rolled down with two passes of a 2.04 kg hard rubber covered roller. The panel is left at room temperature for 30 minutes.
For room temperature shear (RT), the panel is hung two degrees from the vertical plane to prevent peeling of the sample from the plate, and the free-hanging end of the adhesive coated strip is formed into a loop. A 500 or 1,000 gram weight is hung from the loop and a timer is started. The Static Shear Value is recorded in minutes. If no failure occurs after 10,000 minutes, the test is stopped.
If samples pass the room temperature static shear test, an elevated temperature test may be run. The panel is hung at two degrees from the vertical in an air circulating oven (that has been preheated to 70xc2x0 C.) for 10 minutes to allow the sample to come up to temperature. A 500 or 1,000 gram weight is then hung on the loop and a timer is started. The Static Shear Value is recorded in minutes and the test is stopped if no failure occurs after 10,000 minutes.
T-Peel
A double coated tape sample is placed between two strips of 25 mmxc3x97200 mm xc3x970.125 mm anodized aluminum, leaving an adhesive free 25 mm tab at each end of each aluminum strip. The assembly is rolled down with a 6.8 kg roller with one pass forward and one pass backward. The assembly is conditioned at room temperature for 1-2 hours. The tabs are bent back at 90xc2x0 in opposite directions and respectively clamped in the upper and lower jaws of a tensile testing machine. The jaws are separated at 30 mm/minute. The force required to pull apart the tabs is recorded in Newtons/Decimeter (N/Dm).
The percentage (%) conversion is as described above using refractive index.